U.S. Pat. No. 1,856,303 discloses a ceramic mix of 1-10% of a metal with bauxite, kaolin, and/or clay. The metal is finely divided and can be aluminum or magnesium, alone or as a mixture or alloy. Iron powder, ferroaluminum, and silicon are also mentioned. The metal is said to control shrinkage. The patent states that the metal, particularly aluminum powder, oxidizes below the vitrification point of the ceramic body. Other ceramic components include diaspore, gibbsite, sillimanite, cyanite, andalusite and mullite. The mix contains no polyolefin or plasticizer at any stage. The products differ from those of the instant invention.
U.S. Pat. No. 3,351,495 discloses preparation of a battery separator starting with a uniform mix of polyolefin, filler, and plasticizer. The filler can be carbon black, coal dust, graphite; metal oxides and hydroxides such as those of Si, Al, Ca, Mg, Ba, Ti, Fe, Zn, and Sn; metal carbonates such as those of Ca and Mg; minerals such as mica, montmorillonite, kaolinite, attapulgite, asbestos, talc, diatomaceous earth and vermiculite; and a number of other salts and compounds. The mix contains no metal and is not fired.
U.S. Pat. No. 3,526,485 discloses sintered ceramic ware comprising alumina, zirconia, and/or beryllia, with dispersions of titanium and/or zirconium alloys. The metals improve resistance to thermal shock. The mix uses at least 5% metal and at least 65% ceramic oxide component. Dense non-porous bodies, such as cutting tool bits, are made by sintering in a reducing atmosphere or in a vacuum. The metals are not oxidized. The patent refers to prior art in which dispersions of iron, chromium, molybdenum, or tungsten were similarly used.
U.S. Pat. No. 3,706,583 discloses addition of tungsten or molybdenum particles to yttria-, magnesia-, or calcia-stabilized hafnia matrix to improve strength and shock resistance in the sintered body. The result is dense and nonporous. Sintering is done under vacuum, and the metal particles are not oxidized. No polyolefin or plasticizer is involved.
U.S. Pat. No. 3,904,551 discloses a process for making a porous ceramic monolith such as an auto exhaust catalytic converter. A three-component mix is used which comprises a ceramic component, a polymer, and a plasticizer. The ceramic component is a sinterable material such as alumina, spodumene, mullite, zircon mullite, magnesia-alumina, spinel, cordierite, and aluminum titanate. Cordierite is a preferred material and it has the formula 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2. A plasticizer, such as mineral oil, is extracted with hexane, and a microporous structure is obtained. No metal is used in the mix.
U.S. Pat. No. 3,953,562 discloses stabilizing ceramic green sheets against dimensional changes by contacting the sheet with a solvent which softens the binder (which can be a thermoplastic polymer) in the ceramic green sheet and thereby permits relief of stress. The ceramic mix as such contains no metal, nor is the final product porous.
U.S. Pat. No. 4,364,877 teaches making a homogeneous mix of alumina plus a small amount of another oxide, with an organic binder, followed by firing in two steps, the first to remove the binder and the second to sinter the particles to make the frit. No metal is involved, and the product is dense and impervious, not porous.
Australian Pat. No. 277,981 discloses a three-component mix of polyolefin, ceramic filler, and plasticizer. The plasticizer can be extracted. No metals are disclosed, nor is the mix fired or otherwise heated.
British Pat. No. 1,044,502 discloses mixes of polyolefin, ceramic filler, metal (Al or Pb), and plasticizer to make film or filament. The plasticizer can be extracted with a solvent such as petroleum ether to give a porous film. Heating or firing is not taught.
British Pat. No. 1,438,961 discloses a three-component mix of olefin polymer, finely divided sinterable metal, and plasticizer. The plasticizer which is typically a hydrocarbon oil is extracted with a solvent such as hexane, then the structure is heated to remove most or all of the olefin polymer, and finally the structure is fired to sinter the metal particles. No ceramic materials are disclosed.
European Patent Appln. Pub. No. 0169067 (Lanxide Corp.), U.S.A. priorities of July 20, 1984 and June 25, 1985; and U.S. Pat. Nos. 3,870,776 and 3,953,562 teach conversion of molten aluminum to alumina with the aid of a catalyst in a pool of molten aluminum. Such a process is different from the present invention.
I am also aware that certain rocket fuel propellants comprise finely divided Al and ammonium perchlorate oxidant in polybutadiene binder. Such compositions, besides lacking my ceramic filler and plasticizer, would be totally unsuitable for the uses of my products, since they would ignite explosively on firing, leaving a dispersed powder of aluminum oxide.
Addition of aluminum powder to cold pressing powders for ceramic green bodies is known in the industry. This is done for the purpose of improving release from the die or mold, and addition is normally at the level of less than 1%. Such green bodies when fired yield nonporous ceramic shapes.
I am aware of a prior formulation prepared in the laboratories of the assignee of this invention, which contained, for example, 1.4 wt % polyethylene, 93.8 wt % lead, and 4.8 wt % mineral oil. The product was non-porous and was not fired. A metal-free mix was also formulated with lead oxides.
In firing greenware prepared as stated in U.S. Pat. No. 3,904,551, the product tends to shrink. The instant invention reduces such shrinkage by the addition of finely divided metal to the starting mix. X-ray examination shows that the added metal oxidizes substantially during firing. It is believed that the oxidizing of the metal causes expansion into the interstices between the particles of ceramic filler, thereby holding the filler particles apart and thus preventing shrinkage, while simultaneously the oxidizing metal causes bonding between the particles of the ceramic filler. In addition to contributing to bonding in the manner just described, the metal particles also help in the final definition of the pores or cavities of the fired structure. The greenware is, of course, already porous when it is placed in the kiln, because of the prior removal of plasticizer by leaching or by heating; this porosity is largely retained during firing. Also, additional porosity is created during firing by the combustion of polyolefin. Thirdly, during firing, the constituent metal particles may provide additional pores. The conventional, commercial variety of aluminum and other metal powders are made by an atomizing process which results in globular or spheroidal particles. When these materials are used in the present invention a unique type of porosity is obtained. When the greenware is fired, the spheroidal metal particles disappear and they are replaced in the sintered ware by spheroidal pores. These atomizedmetal-generated pores are believed unique. So far as I can determine, such pores are not found in sintered ware fired from any other type of greenware.
These atomized-metal-generated pores are largely open pores, and they interconnect with adjoining pores or passageways throughout the porous structure. The atomized-metal-generated pores are roughly spheroidal or ellipsoidal in shape. Their uniqueness lies in the fact that the "skin" or boundary defining a given pore is made of extremely small metal oxide multifaceted grains, generally contiguous, and typically 0.2 to 3 microns in diameter. The skins of the atomized-metal-generated pores are what makes the pores different from prior art pores. These skins or boundary layers result when the greenware is fired, and form when the atomized metal particles oxidize. These unique metal-oxide-walled pores lie between the particles of ceramic filler and help to bond the ceramic particles. As already mentioned, there will be some oxidized metal not a constituent part of a pore skin or boundary, and this material will typically lie between ceramic particles and will also bond same. The atomized-metal-generated pores can vary in size and shape. A pore diameter in the range of 0.3 to 10 microns is typical. So far as I can determine, the structure that I have just described is novel. The structure is a sintered porous ceramic structure comprising open pores where the skins of the pores being made of minute grains of metal oxide, and where the pores lie in a matrix of porous ceramic filler with the interstices of the filler containing metal oxide. Typically, the filler itself is a metal oxide.
As regards the above mentioned atomized-metal-generated porosity, while I do not wish to be bound to any particular mechanism, I believe that this pore formation involves evaporation of the metal at temperatures below the melting point, possibly enhanced by the exotherm of oxidation, which takes place at that time on the surface of the metal particle. This metal vapor condenses on the surrounding ceramic filler particles, which do not contain metal and, therefore, are colder. As heating continues, this metal oxidizes and adds to the grains of ceramic, surrounding the metal particle. This process is believed to continue until the metal particle is consumed. The oxides formed on the surface of the metal particle during this process are believed to add to the ceramic grains, causing grain growth by a process well-known in ceramics, where, during sintering, smaller grains are absorbed onto larger grains. The end point of the process is when all the metal is consumed by either the evaporation-condensation-oxidation-sintering process or by the oxidation-sintering process, leaving a pore where the metal particle was, said pore being surrounded by a region of greater density than that of the average for the overall body. This mechanism probably applies regardless of the shape of the metal particles. However, in the case of atomized spheroidal metal particles, the pore is spheroidal and is lined with a skin of grains as previously described.
Unless otherwise stated, the metal powders used in the following examples are commercially available atomized varieties.